[A] Nonetheless, the 1997 - 1998 El Nino Was an unusual one. It developed so rapidly that every month between June and December 1997 set a new monthly record high for sea surface temperatures in the eastern equatorial Pacific. Anomalies (that is, deviations from normal in December 1997 were the highest ever recorded along the Equator in the eastern Pacific. Moreover, before 1997 - 1998, the previous record setting El Nino occurred in 1982 - 1983. These two "super El Ninos" were separated by only 15 years, compared with a typical 30 -40 year gap between such events earlier in the 20th century.[B] Identifying why it was so strong challenges our understanding of the physical mechanisms responsible for El Nino. This is more than simply an academic question: the 1997 - 1998 El Nino severely disrupted global weather patterns and Pacific marine ecosystems, and by one estimate caused $ 33 billion in damage and cost 23,000 lives worldwide.[C] There were warnings of a coming El Nino before it occurred But although many computer forecast models predicted that 1997 would be warm in the tropical Pacific up to three season in advance, none predicted the rapid development or ultimate intensity of the event before it began. Clearly we have much to learn from this experience. El Nino, Spanish for "the child" (and specifically the Christ child), is the name Peruvian fisherman gave to coastal sea - temperature warmings that first appeared around Christmas time. Now El Nino more generally refers to a warming of the tropical Pacific basin that occurs roughly every three to seven years in association with a weakening of the trade winds. The opposite side of El Nino, La Nina, is characterized by stronger than normal trade winds and unusually cold sea - surface temperatures in the tropical Pacific. Both El"Nino and La Nina are accompanied by swings in atmospheric pressure between the eastern and western Pacific. These swings are known as the Southern Oscillation. These phenomena are collectively referred to as ENSO or El Nino/ Southern Oscillation.[D] Several factors may have contributed to the strength of the 1997 -1998 El Nino. One is chaos, which some theories invoke to ac count for the irregularity of the ENSO cycle. Nonlinear resonances involving ENSO and the seasonal cycle have received special attention, but other Chaotic interactions may affect ENSO as well.[E]The general mechanisms underlying the ENSO involve large -scale ocean -atmosphere interactions and equatorial ocean dynamics. But each El Nino and La Nina is unique in the combination of its strength, ’duration and pattern of development. Irregularity in the EN SO, cycle can be seen both in the record dating back to the middle of the 19th century, and in other supporting data, such as lake sediments, coral growth rings and tree rings, going back hundreds or even thousands of years. So in principle, it should not be surprising that an unusually strong El Nino occurs every so often.[F] In 1997 - 1998, events possibly acted together to produce an extraordinarily strong El Nino simply dud to the underlying tendency towards chaos in the climate system.[G] Just under a year ago, a sharp drop in equatorial Pacific sea - surface temperature indicated the end of the 1997 - 1998 El Nino. Called by someone "the climate event Of the century", it was by several measures the strongest on record. 46